1. Field of the Invention
The present invention relates to a document reading apparatus having a white light-emitting diode (LED) as a light source.
2. Description of the Related Art
Recently, it has been discussed to include a color image sensor and a monochrome image sensor in a document reading apparatus used in a copying machine in Japanese Patent Application Laid-Open Nos. 2002-111968 and 2008-35561.
Japanese Patent Application Laid-Open No. 2002-111968 discusses a technique in which a document image is read quickly during a monochrome mode, by reading the image with R, G, and B sensors during a color mode and reading the document image with a monochrome sensor during the monochrome mode.
Japanese Patent Application Laid-Open No. 2008-35561 discusses a technique in which the document image is read by a light-receiving element array that includes an optical filter during a first copying mode and read by a light-receiving element array which does not include an optical filter during a second copying mode.
Further, recently, to reduce power consumption, it has been proposed to use a white LED as a light source for color reading in the document reading apparatus (e.g., see Japanese Patent Application Laid-Open No. 2005-236530). To reduce costs, there are some white LEDs that use a blue LED and a yellow light-emitting fluorescent material (phosphor) (e.g., see Japanese Patent Application Laid-Open No. 2000-49374).
However, if the white LED using a blue LED and a yellow light-emitting fluorescent material (phosphor) is used as the light source for a document reading apparatus that includes a color image sensor having an optical filter and a monochrome image sensor that does not have an optical filter, the following problems occur.
As illustrated in FIG. 2, the spectral characteristics for a white LED that includes a blue LED and a yellow light-emitting fluorescent material (phosphor) have a specific wavelength region (around 450 nm) in which the luminous intensity is higher than those for other wavelength regions. On the other hand, as illustrated in FIG. 11, the standard relative visibility characteristics (International Commission on Illumination (CIE) standards) for a person under a daylight light source D65 (JIS Standards (JIS-Z8720)) have a different peak wavelength region (around 555 nm) to the peak wavelength region of the white LED.
Therefore, the monochrome image sensor that does not have an optical filter reads the document image in a color, which is different from the color that can be seen under normal daylight color. In practice, the monochrome image sensor reads the document image as luminance information. Therefore, the monochrome image sensor reads the document image with the luminance (density) for each color in the document image differently from when seen under normal daylight color. This can cause problems in the density reproducibility of the document image.